The reproduction of color was first achieved by Scottish physicist James Maxwell in the mid 1850's. Maxwell photographed a scene three times, once through a red filter, once through a green filter, and once through a blue filter. These black-and-white negatives were contacted to produce positives that were then mounted as slides. Each slide was placed in a different projector and the images were focused together on a screen. A red, green, or blue filter was placed over the lens of each respective projector, thus producing a color image on the screen.
The first single film image for color photography was produced by Louis Ducos du Huron in France in the late 1860's. In his system, the image on a black-and-white panchromatic emulsion was broken up by a series of red, green, and blue transparent dots or lines that formed a screen in front of the emulsion. The dots and lines were so small that they could not be resolved by the eye. After exposure, the film was reversal-processed to yield a colored positive transparency. The additive-color transparency is still used by the Polaroid Corporation with their 35-ram Polachrome slide process.
The development of the subtractive color systems was also pioneered by du Huron. He suggested making separation negatives through red, green, and blue filters, then making positive transparencies from each, dyeing them with colors that absorb each respective primary color (i.e. cyan, magenta, and yellow). This subtractive method is difficult to use because it requires the accurate registration of the colored positives or the accurate registration of images from dyed positive matrices. The solution was a three-emulsion film, each layer made sensitive to a different color (red, green, or blue) and then dyed a different color (cyan, magenta, or yellow) in processing. The first successful film of this type was Kodachrome, introduced by the Eastman Kodak Company in 1935.
Printed color reproduction is based on many of the same principles as film color reproduction. Instead of a continuous image, allowed by the film medium, a series of dots are printed on a substrate. These dots are printed in the subtractive primary colors of cyan, magenta, and yellow. Additionally, black is used to adjust the contrast of the image. In the subtractive process, a white substrate is used and red, green, and blue are essentially subtracted to achieve black. By contrast, in the additive system, a black background (i.e. a blank TV screen) is used, and red, green, and blue are added to achieve white. In the additive system the following combinations create the following results:
Red+Green=Yellow PA1 Red+Blue=Magenta PA1 Green+Blue=Cyan PA1 Red+Green+Blue=White PA1 White+Yellow+Cyan=Green PA1 White+Magenta+Cyan=Blue PA1 White+Magenta+Yellow=Red PA1 White+Yellow+Magenta+Cyan=Black
In the subtractive process, the following is true:
Moreover, each subtractive primary color when added with white produces that same subtractive primary color.
The objective in printing is to produce yellow, magenta, and cyan printing plates that are negative records of the amounts of blue, green, and red in the original. This is achieved by first photographing the original, in turn, through blue, green, and red filters. These films may then be converted into a halftone dot image suitable for a given printing process. The films are then used to make the image carriers, which may be plates, cylinders, or stencils. Each plate is inked with its appropriate ink, which is then transferred to a white substrate.
The image produced is largely dependant upon dot size and orientation. Orientation is defined primarily by the screen angle of the dot. The screen angle is the angle at which the rulings of a halftone screen are set when making screened images. In other words, the screen angle of a dot is the angle of the line which bisects the often elliptical dots. Standard screen angles have been established for various colors of dots: Magenta (45.degree.), Cyan (75.degree.), Yellow (90.degree.), Black (105.degree.). The interaction of screen angle, color, and dot size effect the quality of the reproduction.
Printing metallic colors, such as metallic gold and metallic silver, poses additional problems. Gold has typically been treated as a shade of yellow, while silver has been treated as a shade of gray. Thus the brilliance of these colors is diminished by the blending of hues which occurs in a four color printing system.
A system known as Metallic Integrated Printing Process (MIPP) has been developed for the reproduction of metallic colors by Eckart-Werke Metal Pigments and Powders of Furth, Bayern, Germany. This system requires numerous steps. First, a designer marks-up the artwork to be copied to designate those areas where the MIPP system is required, i.e. metallic colored areas. Next, a conventional four color separation is produced of the artwork. Each separation is then compared to the original artwork to see which separation gives the best representation of the metallic colors. Based on the object color in the original photograph and the color requirements of the final print, a determination is then made whether gold or silver is required. Most shades of gold can be obtained from silver and yellow. However, a high percentage of yellow on silver greatly reduces the metallic brilliance. In addition, silver has a grey value of approximately 30% that tends also to reduce the metallic brilliance and thereby dirty colors.
After the four color separations are made, two separations used to print the metallic inks must be developed from two of the four separations. Typically the cyan or black separation will give the best basis for developing the silver separation and either the yellow or magenta for the gold separation. The selected separations are then duplicated to become the gold and silver separations. These separations may require modification to remove image areas where a metallic effect is not required. Comparison with the original transparency may indicate the need to enhance some image areas so as to improve the final metallic effect. The MIPP system anticipates the softening of mask edges of the metallic colors to avoid sharp cut-out effects when the final result is printed. In practice, the task of softening of mask edges can be handled using electronic image processing equipment.
With the MIPP systemic, a screen angle must be freed for each of the metallic inks to avoid problems of screen clash and resulting moire effects. This can be accomplished by using achromatic or Under Color Removal, ("UCR") color separation techniques where the process color with the lowest value is eliminated in favor of black. UCR involves the technique of reducing the cyan, magenta, and yellow content in neutral grey shadow areas of a reproduction and replacing them with black ink so that the reproduction will appear normal but will use less process color ink. (From the Complete Color Glossary by Miles Southworth, Thad McIlroy and Donna Southworth, Copyright 1992; Published by The Color Resource, Livonia, N.Y. ISBN 1-879847-01-9). Often the cyan will have the lowest value and is the color to eliminate. Since both gold and silver have a process color value, the four conventional separations will need to be modified if the finished print is not to look over-colored or dirty. For example PANTONE 873, the MIPP gold standard, has a process color value of approximately 65% yellow, 25% magenta and 5% cyan. So if the gold areas are to look realistic these colors must be reduced proportionately. The separations may also require modification as the metallic inks have a grey scale value and a failure to take this into account may result in a dirtying of the final colors due to a reduction in their metallic brilliance.
A MIPP image is printed using standard screen angle intervals of 30.degree. or 15.degree.. The screen angle used for a metallic ink is the same as that for the process colors eliminated in favor of a metallic ink. The MIPP system may use different dot shapes to reduce the risk of screen clash. A round dot, with no preferred direction, is typically used for the metallic ink, while an elliptical dot works for the standard process inks. The color standards chosen for MIPP come from the PANTONE System of matched metallic inks, with PANTONE 873 as the gold standard and PANTONE 877 as the silver standard.
Because metallic inks are opaque, they are normally printed before the transparent process colors. But with MIPP the sequence is changed slightly so that the first three colors down are silver, yellow, and gold, respectively. The remaining three process colors are printed in any order. The first three colors, in this order, are very important if the finished print is to look realistic. The use of yellow on silver is necessary to obtain yellow, green and orange metallic effects. Yellow, under gold, is also necessary to maintain the correct tonal values in the highlight areas. Yellow, printed in this way, provides a transition from gold to non-metallic pans of the image. On the other hand, if yellow is printed on top of the gold, there is a loss of metallic sheen without any compensating color benefit.
In summary, the MIPP system presents several disadvantages. First, it requires excessive handwork to create the color mash. Second, the MIPP system requires the elimination of one of the subtractive process colors to free up a screen angle for a metallic color. Third, the MIPP system only allows the printing of four screened colors in any given area. Last, the PMS 873 standard gold ink used by the MIPP system is a dirty, or less brilliant gold ink. This dirty look limits the gold color reproduction to the inherent dirty look even if no other color ink is printed in that area. This dirty look also necessitates additional color correction of the sub tractive primaries. Therefore, a need exists for a printing process which maximizes the appearance of metallic colors. Such a process should allow the use of six colors printed at four screen angles. Moreover, such a process should not limit the number of colors in any given area to four as with the MIPP System.